The ability to reduce the surface tension of water is of great importance in waterborne coatings, inks, adhesives, fountain solutions and agricultural formulations because decreased surface tension translates to enhanced substrate wetting in actual formulations. Surface tension reduction in water-based systems is generally achieved through the addition of surfactants. Performance attributes resulting from the addition of surfactants include enhanced surface coverage, fewer defects, and more uniform distribution. Equilibrium surface tension performance is important when the system is at rest. However, the ability to reduce surface tension under dynamic conditions is of great importance in applications where high surface creation rates are utilized. Such applications include spraying, rolling and brushing of coatings or spraying of agricultural formulations, or high speed gravure or ink-jet printing. Dynamic surface tension is a fundamental quantity which provides a measure of the ability of a surfactant to reduce surface tension and provide wetting under such high speed application conditions.
Traditional nonionic surfactants such as alkylphenol or alcohol ethoxylates, and ethylene oxide (EO)/propylene oxide (PO) copolymers have excellent equilibrium surface tension performance but are generally characterized as having poor dynamic surface tension reduction. In contrast, certain anionic surfactants such as sodium dialkyl sulfosuccinates can provide good dynamic results, but these are very foamy and impart water sensitivity to the finished coating.
In addition to the development of high-performance surfactants, there is considerable interest in the industry in surfactants with improved environmental characteristics. Environmental concerns have led to an increased use of environmentally compatible surfactants as alternatives have become available. In addition, the use of less favorable products, such as alkylphenol ethoxylate (APE) surfactants, has declined. This is, in part, due to the poor environmental characteristics of APE surfactants, such as incomplete biodegradation and a suspicion that they may function as endocrine mimics. The demand for high-performance, eco-friendly surfactants has stimulated efforts in new surfactant development. From this work a new family of surfactants, referred to as alkyl polyglycoside (APG) surfactants, has emerged as a readily biodegradable, environmentally-friendly alternative to conventional surfactants. These materials, however, can be foamy and thus, are not suitable for a variety of coating, ink, adhesive and agricultural applications where the generation of foam is undesirable. Moreover, many APG surfactants possess poor color characteristics and are solids or pastes. This latter property complicates handling and necessitates the formation of blends which contain significantly less than 100% active ingredient. Thus, not only is it desirable to obtain surfactants which exhibit excellent surface tension reducing capabilities and low foam under dynamic application conditions, but it is also highly desirable that such new surfactants are environmentally-friendly, are liquids and possess little or no color.
There is a need for surfactants which exhibit good equilibrium and dynamic surface tension properties, are low-foaming, are low viscosity liquids to facilitate handling, have low color and low odor characteristics and would be widely accepted in the waterborne coating, ink, adhesive, fountain solution and agricultural formulation industries. Moreover, since there is substantial interest in the development of environmentally-friendly surfactants, an essential attribute would be that these surfactants not only possess the aforementioned desired performance attributes but also are derived from naturally occurring compounds or their synthetic equivalents or possess favorable biodegradation and toxicity properties.
The importance of reducing equilibrium and dynamic surface tension in applications such as coatings, inks, adhesives, fountain solutions and agricultural formulations is well-appreciated in the art.
Low dynamic surface tension is of great importance in the application of waterborne coatings. In an article, Schwartz, J. “The Importance of Low Dynamic Surface Tension in Waterborne Coatings”, Journal of Coatings Technology, September 1992, there is a discussion of surface tension properties in waterborne coatings and a discussion of dynamic surface tension in such coatings. Equilibrium and dynamic surface tension were evaluated for several surface active agents. It is pointed out that low dynamic surface tension is an important factor in achieving superior film formation in waterborne coatings. Dynamic coating application methods require surfactants with low dynamic surface tensions in order to prevent defects such as retraction, craters, and foam.
Efficient application of agricultural products is also highly dependent on the dynamic surface tension properties of the formulation. In an article, Wirth, W.; Storp, S.; Jacobsen, W. “Mechanisms Controlling Leaf Retention of Agricultural Spray Solutions”; Pestic. Sci. 1991, 33, 411-420, the relationship between the dynamic surface tension of agricultural formulations and the ability of these formulations to be retained on a leaf was studied. These workers observed a good correlation between retention values and dynamic surface tension, with more effective retention of formulations exhibiting low dynamic surface tension.
Low dynamic surface tension is also important in high-speed printing as discussed in the article “Using Surfactants to Formulate VOC Compliant Waterbased Inks”, Medina, S. W.; Sutovich, M. N. Am. Ink Maker 1994, 72 (2), 32-38. In this article, it is stated that equilibrium surface tensions (ESTs) are pertinent only to ink systems at rest. EST values, however, are not good indicators of performance in the dynamic, high speed printing environment under which the ink is used. Dynamic surface tension is a more appropriate property. This dynamic measurement is an indicator of the ability of the surfactant to migrate to a newly created ink/substrate interface to provide wetting during high speed printing.
U.S. Pat. No. 5,098,478 discloses water-based ink compositions comprising water, a pigment, a nonionic surfactant and a solubilizing agent for the nonionic surfactant. Dynamic surface tension in ink compositions for publication gravure printing must be reduced to a level of about 25 to 40 dynes/cm to assure that printability problems will not be encountered.
U.S. Pat. No. 5,562,762 discloses an aqueous jet ink of water, dissolved dyes and a tertiary amine having two polyethoxylate substituents and that low dynamic surface tension is important in ink jet printing.
A variety of esters of malic acid (2-hydroxy-butanedioic acid), also called malates, are known. Malic acid itself is used primarily as an additive in beverages, candy and food. The commercial form (DL-malic acid) is produced from maleic anhydride and is classified as GRAS (Generally Recognized As Safe) by the U.S. Food and Drug Administration. In addition, the naturally occurring form (L-malic acid) is found in many fruits at low concentrations.
DE 3 011 645 A1 discloses the use of mixed esters of hydroxycarboxylic acids as dispersion aids for the aqueous suspension polymerization of vinyl chloride.
U.S. Pat. No. 3,927,073 discloses esters of dicarboxylic acids with polyhydroxy tertiary amines as both a detergent and a fabric softening agent.
DE 19 621 681 A1 discloses aqueous pearl luster concentrates comprising esters of polyvalent carboxylic acids and/or hydroxycarboxylic acids with fatty alcohols in conjunction with emulsifiers and polyols. The mono- or di-esters of C6 to C22 alcohols were used to impart a pearl luster to “surface active agents” for use in hair shampoo and manual dishwashing detergents. Among the suggested acids is malic acid.
U.S. Pat. No. 5,695,679 discloses dishwashing detergent formulations comprising esters of mono- or polycarboxylic acids and mono- or polyhydric alcohols as “organic silver coating agents”. Malic acid is listed among the carboxylic acids.
U.S. Pat. No. 2,925,352 discloses derivatives of malic acids as plasticisers for water-insoluble thermoplastic organic film-forming polymers. Example 4 shows a film-forming dope comprising 100 parts cellulose acetate, 12.5 parts of diisobutyl malate, 400 parts of dioxalane and 27 parts water.
U.S. Pat. No. 2,122,716 discloses hydroxycarboxylic acid esters of C10-C14 alcohols.
C. D. Vaughan and D. A. Rice, J. Dispersion Science and Technology, 1990, 11, 83, show the use of dioctylmalate in an oil-in-water emulsion used to test an equation for the “Required HLB” value for the oil phase in order to obtain a stable emulsion.
Kyotani et al, Sekiyu Gakkaishi, 1988, 31, 382, studied the effect of OH groups on the flow behavior of mono- and di-ester lubricants made from 2-ethylhexanol. The increased viscosity of di(2-ethylhexyl)malate versus di(2-ethylhexyl)succinate illustrates that the latter is a better lubricant. In this case, all liquids were studied neat and not in aqueous media.
U.S. Pat. No. 4,005,189 discloses as a deodorant an ester of an aliphatic mono- or dihydroxycarboxylic acid or an aliphatic mono- or dihydroxy-dicarboxylic acid having 2 to 4 carbons with an aliphatic alcohol having 1 to 6 carbons. Diethyl malate, diisopropyl malate and dihexyl malate are shown.
EP 0 850 935 A2 discloses concentrated solutions of 1,3,5-triazine derivatives and certain esters of carboxylic acids as solvents. A preferred solvent is bis(2-ethylhexyl) malate.
U.S. Pat. No. 5,505,937 discloses a transfer resistant cosmetic composition containing as a low viscosity oil dioctyl malate among the many listed esters.
U.S. Pat. No. 5,702,693 discloses an aqueous liquid composition for removal of gypsum from the skin of a patient comprising a water-miscible organic solvent, an acid and an emollient. The examples show dioctyl malate as an emollient.
U.S. Pat. No. 5,597,576 discloses oil-based transparent gels, i.e., “lipogels”, containing malic acid diesters of C12-13 single-branch fatty alcohols.